In the present specification and claims, the term "liquid crystalline compound" is used as general name for the compounds which exhibit liquid crystal phase and the compounds which do not exhibit liquid crystal phase but are used as component of liquid crystal compositions.
Liquid crystal display devices utilizing liquid crystalline compounds are widely used in such displays as watches, tabletop calculators, and word processors. These liquid crystal display devices utilize the optical anisotropy and dielectric anisotropy of liquid crystalline compounds.
Liquid crystal phase includes nematic phase, smectic phase, and cholesteric phase. Among them, nematic liquid crystal phase is most widely employed for display devices. On the other hand, there have been developed many display modes such as dynamic scattering (DS) mode, deformation of aligned phases (DAP) mode, guest/host (GH) mode, twist nematic (TN) mode, super twist nematic (STN) mode, and thin film transistor (TFT).
Liquid crystalline compounds used in these display modes are required to exhibit a wide temperature range of liquid crystal phase with room temperature being its center, to be sufficiently stable under conditions in which display devices are used, and to have characteristics sufficient to drive display devices. However, no liquid crystalline compounds which satisfy such requirements by a single compound have been found up to now. Accordingly, it is an actual circumstance that several kind or several tens kind of liquid crystalline compounds are mixed to produce liquid crystal compositions having required characteristics. These liquid crystal compositions are required to be stable against moisture, light, heat, and air which usually exist under the conditions in which display devices are used, to be stable against electric field and electromagnetic radiation, and further to be chemically stable against compounds to be mixed. Liquid crystal compositions are required to have a proper optical anisotropy (.DELTA.n) and dielectric anisotropy (.DELTA..epsilon.) depending on the display mode and the shape of display devices. Further, it is important that each component in the liquid crystal compositions has an excellent miscibility with one another.
In recent years, demand for the compounds exhibiting liquid crystal phase at a wider temperature range, particularly for the compounds exhibiting a high transition temperature to isotropic phase is elevated in keeping with diversification of environments in which liquid crystal display devices are used. Besides, in order to expand the size of screen and improve the quality of display, it is preferable that the compounds have a low viscosity. As such compounds, ones expressed by the formula (a), (b), or (c) are disclosed in Japanese Patent Publication No. Sho 62-46527, Japanese patent Publication no. Hei 4-28693, or Japanese Patent Publication No Hei 2-1811, respectively. Also, a compound having an alkenylene group as bonding group and expressed by the formula (d) is known (24th FREIBURGER ARBEITSTAGUNG FLUSSIGKRISTALLE P01 (1995)). ##STR1##
Whereas the compounds of the formula (a) or (b) have a comparatively high transition temperature to isotropic phase, their viscosity can not be said to be sufficiently low. Compound of the formula (c) has a strong smectic property and the miscibility with other liquid crystal materials is poor. Compound of the formula (d) has a problem that the transition temperature to isotropic phase is low.
Therefore, development of liquid crystalline compounds having a wider temperature range of liquid crystal phase, low viscosity, and excellent miscibility with other liquid crystal materials are long-awaited.